1. Technical Field
The present invention relates to an optical coupler and, more particularly, to an optical coupler including an anamorphic lens in combination with conventional focusing optics to improve coupling efficiency between a laser transmitter and an optical fiber.
2. Description of the Prior Art
Many diode lasers, especially 980 nm erbium-doped fiber pump lasers, have an elliptical beam pattern in which one axis is much faster than the other. That is, the numerical aperture in the perpendicular direction may be approximately sin (30.degree.) (referred to as the "fast axis"), while the numerical aperture in the parallel direction may be approximately sin (10.degree.) (referred to as the "slow axis"). This difference in numeral aperture makes it difficult to achieve high coupling efficiencies into a single mode fiber.
Several solutions have been proposed and implemented. One solution involves collimating the laser beam and using prism beam expanders to re-form one axis of the beam before focusing the beam into the fiber. See, for example, the article entitled "Design of beam-shaping optics" by J. Braat appearing in Applied Optics, Vol. 34, No. 15, May 20, 1995, at page 2665. Another solution is to use an anamorphic objective lens placed in front of the laser to reduce the divergence in the fast axis. This solution is discussed in detail in an article entitled "Diffraction Limited Virtual Point Source Microlenses" by S. W. Connely et al appearing in SPIE, Vol. 2383, 1995, at page 252. As discussed in the Connely et al article, the reduction of the divergence in the fast axis results in the beam seeming to emanate from a point source. While the use of such an anamorphic lens with the laser source is a useful solution, the combination has inherently tight alignment tolerances. In particular, the tolerances are on the order of 0.5 micron, resulting in an arrangement that is difficult to implement in a manufacturing environment.